Cover crops and N fertilization affect soil ammonia volatilization and N2O emission by regulating the soil labile carbon and nitrogen fractions

Introducing cover crops in monocropping systems to improve soil health has been broadly adopted worldwide. However, the impacts of different cover cropping systems on soil emissions and related soil fractions and their regulatory mechanism remain elusive. In this study, four cropping systems, i.e., relay intercropping with Orychophragmus violaceus L./cotton (OvC), Vicia villosa Roth/cotton (VvC), a cover crop mixture (Ov and Vv)/cotton (MSC) and a fallow field–cotton (FFC) system, were combined with four N application rates in a two-rotation cycle (2019–2021) to evaluate the effects on soil greenhouse gases (GHGs) emission, ammonia (NH3) volatilization, and labile carbon (C) and nitrogen (N) fractions and their relationships. Compared with the FFC system, the cumulative NH3 volatilization and nitrous oxide (N2O) emissions of OvC are noticeably reduced 13% and 58%, respectively, and those of VvC are comparable, while those of MSC are markedly increased by 15% and 24%, respectively. N fertilization significantly affects soil NH3 volatilization and N2O emissions, and a higher N rate results in more gaseous N emissions. In addition, NH3 volatilization and N2O emission are positively correlated with the soil microbial biomass N (MBN), dissolved organic N (DON), NH4+-N and NO3--N but negatively correlated with microbial biomass C (MBC) and dissolved organic C (DOC). OvC system decreased gaseous N emission by high soil MBC/MBN ratio, Vv as cover crop indirectly promote soil N2O emission through increased soil MBN, DON and MBC content, while cover crop mixtures increase soil labile C and N fractions and enhance gaseous N emission ultimately. Overall, application of N fertilizer and cover crop mixture both promoted soil NH3 and N2O emissions; the relay intercropping nonlegume cover crop Ov noticeably mitigates N2O and NH3 emissions through an increase in the soil MBC and reduce in the labile N, while introduction of the legume cover crop Vv increases N2O emissions but reduces soil NH3 volatilization due to a lower soil MBC/MBN ratio.